In semiconductor manufacturing devices, it is necessary to precisely determine the spatial position of certain parts, which are movable relative to each other, using suitable position-measuring devices. A computer-controlled sequencing control in these devices is then possible based on the position information. Thus, for example, in wafer steppers, the position of mask and wafer must be measured very precisely in all 6 degrees of freedom (6DOF). In conventional methods, this position measuring is mainly performed using several laser interferometers. In the future, it can be expected that the stringent accuracy requirements of the position measuring, accompanied at the same time by increasing traversing speeds of the various parts, will further increase. While, currently, for example, accuracy requirements of a few nm at speeds of approximately 1 m/s may be specified, in the future, subnanometer accuracies at markedly higher speeds can be expected. However, given such high accuracy requirements, it is believed that laser interferometers are not usable as position-measuring devices, since refractive-index fluctuations in the ambient air, even when working with optimal air showers, lead to measured-value fluctuations in the position measuring of several nm.
For this reason, alternative position-measuring devices have already been proposed in equipment of this type. Thus, it is described, for example, in European Published Patent Application No. 1 019 669 to use optical position-measuring devices having so-called cross gratings as a two-dimensional measuring graduation. Systems of this type are minimally influenced by possible fluctuations in the refractive index of the air, and therefore allow position measurements which may be reproduced well.
From the cross-grating scanning, the lateral degrees of freedom X, Y, Rz can be recorded in such systems (X: translation along the X-axis; Y: translation along the Y-axis; Rz: rotation about the Z-axis). Therefore, in order to record all six possible degrees of freedom, and also the additional degrees of freedom Z, Rx, Ry (Z: translation along the Z-axis; Rx: rotation about the x-axis; Ry: rotation about the y-axis) using measuring techniques, a further position measurement is required in the z-direction. For this purpose, European Published Patent Application No. 1 019 669 describes additional distance sensors, e.g., capacitive or contacting probes. However, such distance sensors are believed to be insufficient, given the precision required.
Alternatively, in principle, interferometers may also be used as distance sensors supplying the necessary precision. However, they require significant extra expenditure with regard to housing, signal processing, signal correction, etc. For example, in this case, it is thus difficult to mount air showers which have a laminar flow parallel or perpendicular to the cross grating. However, air showers of this type may be indispensable for sufficient interferometer measuring accuracy at this location. In addition, it is problematic to accomplish the exact time synchronization of the position-measurement values from the cross-grating scanning and from the interferometric distance measuring.
Furthermore, in conventional grating-based position-measuring devices, it is also possible to determine the scanning distance between two gratings in the scanning beam path. In this connection, reference is made, for example, to European Published Patent Application No. 0 448 982.